1. Field of the Invention
The present invention relates to a shaft furnace for directly reducing the ores by the counter-flow of ores which descends in the furnace and a reducing gas which rises in the furnace.
2. Description of the Prior Art
The production of reduced iron by the shaft furnace of this type has long been placed in the industrial applications. To increase the productivity by the shaft furnace of this system, it is necessary to essentially increase the rate of reduction reaction. For this purpose, the reduction temperature may be elevated or the reaction pressure may be increased. As for the method of increasing the reacting pressure, i.e., as for the operation system under elevated pressure, a pilot plant has been already developed and there has been reported that the reduction time was remarkably reduced by the operation under the pressure of 4 kg/cm.sup.2 G. With the operation of the system under high pressure, however, a specially designed sealing mechanism must be employed to cope with the increased pressure, requiring increased cost for the construction of facilities.
On the other hand, the method of increasing the productivity by raising the reduction temperature is advantageous as compared with the high-pressure operation system because it does not require increased cost for the construction of the aforementioned facilities. This method, however, presents such a problem that the burden materials are often clustered with each other due to elevated temperature.
That is, ores which are the burden materials accumulate and gradually descend through the shaft furnace. In a high-temperature zone at the lower portion of the reduction zone, therefore, the clustering develops such that ores adhere with each other due to metallurgical change as a result of the pressure of ores (pressure which generates in the ore layer due to the self weight of the ores) and reduction of ores, giving rise to the occurrence of deflection in the flow of reducing gas as well as the occurrence of hanging. In extreme cases, therefore, it becomes difficult to continue the operation, presenting a serious problem in producing the reduced iron with the shaft furnace of this type.
To avoid the clustering, it has been attempted to use ore pellets admixed with small amounts of limestone (refer to the specification of U.S. Pat. No. 3,957,486). In this case, however, limitation is imposed on the ores which are to be processed.
Furthermore, to prevent the clustering with the conventional methods, it was often attempted to provide agitating wings, baffle plates or burden feeders at the lower portion of the furnace to destroy the clustering (refer, for example, to U.S. Pat. No. 3,558,118).
Thus, there had been no effective method for preventing the clustering in the reduction zone.
In view of such circumstances, the inventors of the present invention have conducted research in relation to the phenomenon of clustering, and have previously announced the results in a paper entitled "Reduction Properties of Raw Materials for Direct Reduction Shaft Furnace", Iron and Steel, 1977, Vol. 63, No. 14, pp. 2269-2277, by Dentaro Kaneko et al., and in a paper entitled "Clustering Phenomena during Iron Oxide Reduction in Shaft Furnace", Iron and Steel, 1978, Vol. 64, No. 6, pp. 681-690, by Dentaro Kaneko et al.
In a series of these studies, it was clarified that the clustering which takes place during the reduction process of a shaft furnace is a sintering phenomenon by diffusion of solid metallic iron that is formed by the reduction, and that if microstructurally considered, the clustering develops and grows by the intertwining of fibrous metallic iron that is formed by the reduction. It was further clarified that the clustering is caused by the three principal factors; i.e., reduction temperature, compressive load which acts upon the burden raw materials, and properties of the burden raw materials. At the same time, various effects by these factors upon the clustering were also clarified.
The present inventin was accomplished based upon the results of such studies and various experiments and simulation tests performed in relation to the formation of cluster, reduction conditions of ores in the furnace, flowing conditions in the furnace, and the like.